The advantages of laser welding over the conventional welding processes in terms of flexibility, speed and weld quality are now well recognised. Indeed many applications of lasers to welding in industry have already been accepted. As more welding systems are being installed by industry, the demand increases for the development of in-process techniques to monitor and control the process quality. This is necessary since weld quality is often affected by the instability of plasma formation during laser welding and instabilities of laser power density.
A multitude of techniques have already been investigated for detecting the weld quality during processing. These include the use of an acoustic mirror which detects the high frequency component of back reflected laser beam from the beam/material interaction zone; an acoustic probe which detects the shock wave generated by the plasma and vapour; an acoustic workpiece which detects the workpiece internal stress waves generated during laser welding; a photo-electric sensor for detecting plasma intensity, a probe laser for detecting melt ripple and plasma diagnosis; a pyrometer for detecting the temperature near the melt pool, and a video camera for monitoring the interaction zone shape.
The known systems and their disadvantages and shortcomings are summarized in the following Table 1.
TABLE 1 __________________________________________________________________________ Previous in-process laser welding monitoring techniques METHODS PRINCIPLE COMMENTS __________________________________________________________________________ Acoustic mirror mirror thermal expansion fast, coaxial, caused by laser backreflection poor S/N ratio probe melt pool vapour or plasma good but off-axial shock wave + laser reflection temperature dependent workpiece stress wave caused by informative contact material structure changes and position dependent Photo- up light emission from fast, cheap but not electric plasma, vapor, melt pool coaxial and noisy below light emission from not very informative bottom side of melt pool and difficult to place Laser refl. probe laser beam reflection fast but off-axial probe from the melt pool trans. probe laser angle change good but off-axial through plasma and expensive Pyrometer Temperature near good but direction melt pool dependent Vision system Image of plasma or informative but weld slow and expensive pyro- up laser beam back reflection coaxial, but sensor electric using a beam spitter easy to be damaged sensor below laser beam through the works only when open key hole there is an opening __________________________________________________________________________
In all of these known methods, a commercial sensor has to be used and the repeatability of these sensing techniques depends on the sensor quality, position of the sensor (such as a photo-diode), thermal stability of the sensor and contacting stability of the sensor (such as acoustic sensors). For multi-dimensional laser welding with a robot arm, for example, most of the above techniques are inadequate either because of the direction or position dependence of the sensor or the bulky body of the associated sensing system. Pulse-weld monitoring techniques such as those based on ultrasonics and eddy currents also suffer from slow response or poor flexibility for high speed multi-dimensional laser welding. For high speed laser welding, some of the methods, such as video image analysis, may not be fast enough for on-line feedback control.
It is an object of the present invention to provide a means of monitoring the quality of laser material processing made using laser apparatus and involving processes such as laser welding, drilling, cutting and surface treatment. A secondary object is to be able to monitor the position of the laser beam itself.
It is known that, during some types of laser materials processing, when laser beam intensity is high enough, an ionized plasma plume is generated in or near the laser-generated melt pool. It is also already known from an article in the Welding Journal, vol.68, No.6, 1989 pages 230S-235S entitled "An Electrostatic Probe for Laser Beam Welding Diagnostics", that the electron density of the plasma in the region above a laser weld can be established using an electrostatic probe consisting of two metallic electrodes in contact with the ionized gas in said region. The workpiece itself serves as one of the electrodes. The other electrode comprises a thin strip or plate of molybdenum with a small hole in it just large enough to pass the laser beam to the workpiece. All surfaces of the molybdenum strip, except that of the interior of the hole, are covered by an electrically insulating film. The insulated molybdenum strip is disposed so as to lie flat on the workpiece, i.e. in contact with it, and is aligned with the laser so that the laser beam passes through the hole to the workpiece. The electrodes are connected to a DC power supply via a resistor so that the current through the ionized gas can be monitored by measuring the voltage across the resistor. The device disclosed in the article must be in contact with and surround the plasma to form an enclosure to enable the collection of all of the electrons and ions by the application of the voltage applied by the DC power supply between the strip or plate and the workpiece. Its purpose is simply to measure the average and overall electron density and thereby give information about the plasma plume. Since the device must of necessity be in engagement with the workpiece, it is not suitable for monitoring the processing of moving objects on line and in real time. Furthermore, the device described in the article was reported to be incapable of being used for more than one measurement as it is destroyed rapidly by the heat from the workpiece.
The present inventors have noted that, at the high temperatures prevailing during laser processing, the plasma plume expands and a polarized charge is developed in the space between the workpiece and the space around it, and they have now established that the degree of separation of the charge which occurs is related to the prevailing laser processing quality and that a signal can be generated by monitoring this charge separation which can provide useful real time information as to the quality of an on-line process being undertaken.